For example, a solid polymer electrolyte fuel cell employs a solid polymer electrolyte membrane. The solid polymer electrolyte membrane is a polymer ion exchange membrane, and interposed between an anode and a cathode to form a membrane electrode assembly (MEA). The membrane electrode assembly is sandwiched between separators to form a unit cell. In use, normally, a predetermined number of the unit cells are stacked together to form a fuel cell stack.
In the fuel cell, a fuel gas flow field is formed in a surface of one separator facing the anode for supplying a fuel gas to the anode, and an oxygen-containing gas flow field is formed in a surface of the other separator facing the cathode for supplying an oxygen-containing gas to the cathode. Further, a coolant flow field is formed between the separators for supplying a coolant along surfaces of the separators as necessary.
In the case where metal separators of thin corrugated plates are used as the separators, by providing grooves as the fuel gas flow field on one surface of the metal separator facing the anode, ridges as the back side of the grooves are formed on the other surface of the metal separator. Further, by forming grooves as the oxygen-containing gas flow field on one surface of the metal separator facing the cathode, ridges as the back side of the grooves are formed on the other surface of the metal separator.
In the structure, by providing corrugated grooves in a serpentine pattern to form the fuel gas flow field and the oxygen-containing gas flow field, the back surfaces of the grooves are stacked together between unit cells to form a coolant flow field where a coolant flows in a direction different from the flow directions of the fuel gas and the oxygen-containing gas.
For example, according to the disclosure of Japanese Laid-Open Patent Publication No. 2007-141553, as shown in FIG. 11, a plurality of unit cells 1 are stacked together to form the fuel cell stack. Each of the unit cells 1 includes metal separators 3, 4 on both sides of a membrane electrode assembly 2.
The membrane electrode assembly 2 includes an anode 2b, a cathode 2c, and a solid polymer electrolyte membrane 2a interposed between the anode 2b and the cathode 2c. A plurality of fuel gas flow grooves 5 extending vertically in a serpentine pattern are formed on a surface of a metal separator 3 facing the anode 2b. A plurality of oxygen-containing gas flow grooves 6 extending vertically in a serpentine pattern are formed on a surface of a metal separator 4 facing the cathode 2c. 
Grooves 7 are formed on the back of the fuel gas flow grooves 5 of the metal separator 3. Grooves 8 are formed on the back of the oxygen-containing gas flow grooves 6 of the metal separator 4. Therefore, when the unit cells 1 are stacked together, the grooves 7, 8 are overlapped together to form a coolant flow field extending in a horizontal direction between the unit cells 1.